EIGRP Notes Part V

LSA Type 1 that each router creates to represent itself for each area it connects to. The LSDB for an area has one Type 1 LSA per router per area that lists the RID and all interface IP addresses on that router that are in that area. Represents stub networks too.

What is the Network LSA?

LSA Type 2 that is one per transit network. It is created by the DR on the subnet and represents the subnet and router interfaces connected to the subnet.

What is the Network Summary LSA?

LSA Type 3 created by ABRs to represent subnets listed in one area’s type 1 and type 2 LSAs when being advertised into another area. Defines the links in the origin area, and cost, but no topology data.

What is the ASBR Summary LSA?

LSA Type 4 that advertises a host route used to reach an ASBR, like a type 3 LSA.

What is the NSSA External LSA?

LSA Type 7 that is created by ASBRs inside an NSSA area, instead of a type 5 LSA.

What is listed in Type 1 LSAs?

Router’s interface subnet number/mask and interface OSPF cost (in which no DR has been elected).

IP address of the DR and a notation that the link attaches to a transit network (for each interface which a DR has been elected).

Lists the neighbor’s RID, for each interface with no DR, but for which a neighbor is reachable.

What are the two main reasons OSPF uses a DR in a particular subnet?

To create and flood a Type 2 network LSA for that subnet.

To aid in the detailed process of database exchange over that subnet.

What rules do routers follow when no DR exists at the time?

Choose the router with the highest priority.

If tied on priority, choose the router with the highest RID.

Choose a BDR, based on next-best priority, or if a tie, next-best (highest) RID.

How is a Type 2 LSA created?

DR in a subnet creates the Type 2 LSA for that subnet. Then identifies LSA by assigning an LSID for the DR’s interface IP address for that subnet. The type 2 LSA also lists the DR’s RID as the router advertising the LSA.

What is the concept of LSA Type 3?

ABRs don’t forward Type 1 and Type 2 LSAs from one area into another area, and vice versa. ABRs generate a Type 3 LSA for each subnet in one area, and advertises each Type 3 LSA into the other areas. Type 3 LSA appears to be another subnet connected to the ABR that created and advertised the Type 3 LSA.

What does an ABR do with a Type 3 LSA?

The ABR creates and floods each Type 3 LSA into the next area. The ABR assigns an LSID of the subnet number being advertised. It also adds its own RID to the LSA so that routers know which ABR advertised the route. It also includes the subnet mask. The correlation between the advertising router’s RID and the LSID (subnet number) allows the OSPF processes to create the part of the topology.

How do you limit the number of LSAs?

Trick question. You can’t. You can set a maximum number of LSAs learned from other routers with the max-lsa number OSPF subcommand.

When router hits the maximum LSA in max-lsa number it will issue log messages for a time period, repeatedly. In the end, the router will close all neighborships, discard its LSDB, and then starts adding neighbors again.

What is the summary of internal LSA types?

show ip ospf database router – Type 1 Router LSA representing a router. LSID is equal to the RID of the router. Each router creates its own.

show ip ospf database network – Type 2 Network LSA representing a subnet in which a DR exists. LSID is equal to the DR’s IP address in the subnet. Created by the DR in that subnet.

show ip ospf database summary – Type 3 Summary LSA that represents a subnet in another area. LSID is equal to the subnet number and is created by an ABR.

What is the Hello OSPF message type?

Used to discover neighbors, supply info used to confirm two routers should be allowed to become neighbors, to bring a neighbor relationship to a 2-way state, and to monitor a neighbor’s responsiveness in case it fails.

What is a Database Description (DD or DBD) OSPF message type?

Used to exchange brief versions of each LSA, typically on initial topology exchange, so that a router knows a list of that neighbor’s known LSAs.

What is a Link-State Request (LSR) OSPF message type?

A packet that lists the LSIDs of LSAs the sender of the LSR would like the receiver of the LSR to supply during database exchange.

What is a Link-State Update (LSU) OSPF message type?

A packet that contains fully detailed LSAs, typically sent in response to an LSR message.

What is a Link-State Acknowledgement (LSAck) OSPF message type?

Sent to confirm receipt of an LSU message.

What are the OSPF Neighbor States?

Down – no Hellos have been received from neighbor for more than the dead interval.

Attempt – Used when the neighbor is defined with the neighbor command, after sending a Hello, but before receiving a Hello from that neighbor.

Init – A Hello has been received from the neighbor, but it did not have the local router’s RID in it or lists parameters that do not pass the neighbor verification checks. This is a permanent state when Hello parameters do not match.

2Way – A Hello has been received from the neighbor, it has the router’s RID in it, and all neighbor verification checks passed.

ExStart – Currently negotiating the DD sequence numbers and master/slave logic used for DD packets.